In conventional powder processing sometimes referred to as "press and sinter", metal, ceramic, intermetallic, refractory powders and the like are compacted under a very high pressure into bodies or parts having a desired shape and the compacted body is sintered in a furnace to yield the solid shaped product. An inherent problem in this process arises from the fact that the force of the applied pressure experienced by the compacted body decreases rapidly interiorly below its surface due to the friction between the powder particles. Therefore, the density of the compact varies from its surface to its center, resulting in non-uniform shrinkage of the compact upon sintering. Complex shapes are therefor difficult to faithfully reproduce by the conventional powder processing because of the non-uniform changes in shape caused by such non-uniform shrinkage during sintering.
The PIM process has become an important new technology in recent years in the processing of various kinds of molding powder into solid parts. The PIM process combines the plastics injection molding technology for forming desired shapes and the conventional powder processing for converting the molded parts into solid bodies. In the PIM process, the powder is mixed with a binder which forms the essential difference between the PIM process and the conventional powder processing. The binder is solid at room temperature but becomes fluid upon heating to its melting or fusion temperature. Therefore, the mixture behaves as a solid at room temperature but as a fluid upon heating, particularly when under molding pressure. The mixture, while in heated fluid condition, can be easily molded into desired shapes, including those of complex configuration, using plastics-type injection molding equipment instead of simple compacting of the solid powder particles. The solid molded so-called "green parts" have a uniform density throughout due to the fluid behavior of the mixture under heat and pressure. The binder is then typically removed or "debound" from the green part and the debound green part is treated, e.g. by sintering in a furnace, to obtain the final product. The principal advantage of the PIM process is the ability to mold complex parts with a uniform density. These parts, after debinding and further treatment, such as sintering, become the final products requiring little or no additional finishing. The disadvantages of the PIM process are difficult mixing and debinding operations.
The binder plays the key role and largely controls the success of the PIM process. A good binder must possess a number of required characteristics: a very low viscosity when molten for mixing with fine powder particles, reasonable mechanical properties when solidified for adequate green strength, good adhesion to the powder particle surface for easy mixing and good green strength, thermal or chemical properties necessary for easy debinding, ability to hold a given shape without distortion during molding, debinding and sintering, and absence of any significant adverse effects on the final product properties as well as those related to economic and health concerns. Such a combination of characteristics is usually obtained by combining several binder components to form a multi-component binder, such as the common wax-based binders used in combination with thermosetting polymers alone or blended with thermoplastic polymers by M. A. Strivens in U.S. Pat. No. 2,939,199 issued on Jun. 7, 1960, and with thermoplastic polymers in Canadian Patent 615,429 issued on Feb. 28, 1961, wherein the wax component is described as being removable either by vacuum distillation or by solvent extraction.
Although particle size is variable, powder particles used in the PIM process often have particle sizes in the order of microns for good processing behavior. A high powder volume fraction in the range of 40-70% by vol., usually 60-70% by vol., of the part is often desired to achieve good properties in the final products. Such a high powder volume fraction close to the maximum possible packing fraction, coupled with extremely small particle sizes, makes the tasks of mixing and debinding of the binder profoundly difficult. Mixing is accomplished by brute force, usually using a sigma-blade or banbury batch mixer running for many hours, usually at an elevated temperature. Although such mixing operations are expensive, they cause no damage to the mixture as long as the mixture is thermo-mechanically stable at the mixing conditions.
Debinding itself is not only difficult but also often results in undesirable distortion of the part shape, rendering it useless. For example, green parts made of wax-based binders can only be freed of the wax by heating above the melting point of the wax irrespective of the technique used, whether it be pyrolysis, wicking or even solvent leaching. Specifically, a wax cannot be leached away by solvent extraction unless the extraction takes place at a temperature above the melting point of the wax in question and it will be noted that in the Strivens U.S. patent identified above, the solvent extraction is said to be with boiling or running hot solvent or solvent vapor and in the Canadian patent by means of a Soxhlet extractor (wherein the solvent is at its boiling point). Waxes commonly soften in the range of about 40.degree.-60.degree. C.
Since the wax component therefore necessarily softens during the extraction, the green parts become soft and slump during debinding. This problem severely limits the utility of wax-based binders to only small simple parts. It is obviously essential that the binder remain rigid or otherwise be able to support the weight of the part during debinding in order to preserve its shape faithfully and avoid distortion thereof.
It has been suggested that the binder can be constituted of sublimable material and thus capable of removal by sublimation, including freeze-drying and reactive sublimation and, in principle, sublimable binders meet the above requirements. Herrmann in U.S. Pat. No. 3,330,892 utilizes as the binder for molding particles an organic vehicle that is solid at normal room temperatures, has a melting point below about 200.degree. C., and a vapor pressure of at least 1 mm Hg at its melting or fusion temperature and normal ambient pressure of one atmosphere, such as naphthalene and camphor, and thus can be vaporized away by heating to a temperature at which it is volatile. A sublimable material, however, in practice, is subject to important drawbacks. Whether used alone as the binder or blended with a polymer fraction, the binder must unavoidably be at least at its melting or fusion temperature for mixing purposes and since few polymers have melting or fusion temperatures below the melting points of sublimable compounds, which in the Herrmann patent are in the range of about 50-130, except for camphor which melts at 175.degree. C., the mixing/shaping temperature will normally be considerably higher. Thus, mixing and shaping, e.g. injection molding, must take place at at least the binder melting or fusion temperature and at that temperature, significant sublimation of the binder is, by definition, unavoidable. Hence, significant amounts of chemical vapor will escape during mixing and shaping and will deposit on the surfaces of the processing equipment, causing processing difficulties and molding defects. In addition, the proportion of binder to molding particles in the feedstock changes during these stages, making the production of a part of defined composition difficult unless complicated and expensive measures are taken to prevent binder vaporization by enclosing the mixing and molding equipment in a pressurized chamber, as Herrmann indeed recommends. Moreover, even if sublimation can be avoided prior to the debinding stage, measures must be taken to capture and recover the sublimation vapors, especially in view of modern environmental regulations not to mention cost considerations, as Herrmann also acknowledges. Therefore, a sublimable binder is at best an inadequate solution to the problem of providing a satisfactory binder for molding particles.
The above discussion of the problem of shape distortion during the debinding stage is equally applicable to all binder-assisted processing of molding particles by injection molding, extrusion, etc. An important contribution in the powder processing technology will be a binder system which can be easily debound from the molded or extruded or otherwise shaped parts without causing appreciable distortion of their part shapes.
Development of binders have been largely empirical as evidenced by the patent literatures. As identified above, M. A. Strivens in U.S. Pat. No. 2,939,199 teaches composite binder compositions made of wax and thermosetting resins alone or blended with thermoplastic resins. R. E. Wiech, Jr., in U.S. Pat. No. 4,197,118 issued on Apr. 8, 1980, and U.S. Pat. No. 4,415,528 issued on Nov. 15, 1983, forms binder compositions of wax and polyethylene resin. In the -118 patent, extraction of the binder by a solvent either in the vapor phase or in the liquid phase at a temperature at or preferably above the melting point of the binder is suggested while the -528 patent emphasizes special sintering conditions.